A typical microphone comprises a voltage bias element (commonly, consists of an electret), a diaphragm/backplate pair forming a variable capacitor according to an acoustic pressure, and a JFET (junction field effect transistor) for buffering an output signal. An electret condenser microphone comprises an electret on one of the diaphragm and the backplate. A front electret refers to a case where the electret is formed on the diaphragm, and a back electret refers to a case where the electret is formed on the backplate. Commonly, the electret is formed by forcibly injecting a charge into an organic film.
FIG. 1 is cross-sectional view illustrating a conventional condenser microphone assembly using a casing. Referring to FIG. 1, the conventional condenser microphone assembly has a structure wherein a vibrating plate 12 including a diaphragm 12a and a polar ring 12b, a spacer 13, a first base 14, a back plate 15, a second base 16, and a PCB 18 are inserted into a casing 10 having an acoustic hole 10c at a bottom surface thereof, and then an end portion 10b of the casing is curled.
The second base 16 carries out a function of mechanically fixing the inserted internal component as well as a function of transmitting an electrical signal generated from a microphone unit (vibrating plate/back electret) to JFET, an amplifier or a device including an amplifier and an AD converter.
In addition, the PCB 18 presses the second base 16 because a height of the second base 16 is higher than that of the first base 14.
However, when the conventional casing is used, a folded surface 10a between a bottom surface and a sidewall of the casing 10 has a certain curvature due to a limitation in a processing technology. Therefore, the component disposed on the bottom surface of the casing 10 is in contact with a curved surface to be deformed. That is, when the vibrating plate 12 is inserted on the bottom surface of the casing, other components are disposed thereon and the end portion 10b of the casing is curled, a pressure is applied inward to the components by a curling process to bend the polar ring of the vibrating plate 12 due to an edge portion of the vibrating plate 12 touching the curved surface of the folded surface 10a of the casing. Therefore, the diaphragm is deformed, resulting in a degradation of a sensitivity and a frequency characteristic of the microphone.